It finds applications in digital cellular radiocommunication systems. The group calls are provided for, for example, in the ETSI (European Telecommunications Standard Institute) standard concerning the Pan-European GSM Phase 2+ (“Global System for Mobile communications Phase 2+”) wireless telephony system. They also constitute a particularly important aspect of professional systems for radiocommunication with mobiles (PMR—“Professional Mobile Radio”) such as the TETRA (“Trans European Trunked Radio”) system or the TETRAPOL system dedicated in particular to the security forces (police, fire service, etc.), in which these calls are made in half-duplex mode, that is, according to a push-to-talk type communication principle.
Unlike a basic call which concerns only two MTs at most, a group call concerns at least three MTs, no more than one of which is in sending phase (hereinafter, the sending MT) and the others are in receiving phase (hereinafter, the receiving MTs) at a specified instant. The traffic information originating from the sending MT is broadcast by the network in the radio coverage area of the group call, that is, in the cells (a cell being the area of radio coverage by a base station (BS)) in which is located at least one receiving MT involved in the group call. The group call is set up, in a manner known per se, by means of a call setup procedure using Control CHannels (CCH) such as the BCCH (“Broadcast CCH”) in GSM. This procedure is used to allocate an inbound traffic channel (TCH) in the cell in which the sending MT is located, and an outbound traffic channel in each cell where at least one receiving MT involved in the group call is located.
It is known that one of the particular features of cellular radiocommunication systems resides in management of the mobility of the MTs, in particular during a call. Handover for an MT roaming from one cell to another is well managed in the case of basic calls, and is widely used in public cellular wireless telephony systems.
It normally relies on a process of preparatory measurements carried out by the MT and/or by the BS, and, furthermore, a frequency change process based on a dialogue between the fixed network and the mobile terminal. This dialogue can be conducted only if an associated signalling channel is permanently open during the call.
In the earliest analogue wireless telephony systems, such a signalling interchange was conducted in the form of inaudible signalling. It can be offered, in the digital wireless telephony systems, in the form of time slots on the carrier frequency of the TCH dedicated to the transmission of signalling outside of the time slots dedicated to the transmission of speech. The recurrence of these signalling time slots on the carrier frequency of the TCH is designated as signalling channel associated with the TCH. This technique, which is perfectly well suited to basic calls, is no longer in the case of the group calls.
In practice, in the case of the GSM Phase 2+ wireless telephony system, for example, only the handover for the sending terminal can be carried out in the group calls because only this terminal can use the inbound signalling channel associated with the inbound traffic channel that is dedicated to it. However, the receiving MTs do not have, in the inbound direction, any dedicated signalling channel that is likely to be used for this purpose. The broadcast transmission of information in the coverage area of the group call allows in practice for a passage of information only from the network to the MTs located in the cells concerned. The receiving MTs that are roaming to other cells and because of this want to proceed with a handover, must use another technique.
The technique currently used in the TETRA system is based on a process of interrupting and reestablishing a call. It has two drawbacks. On the one hand, it can give rise to collective random access phenomena prejudicial to the stability of the system. When a plurality of MTs that are involved in the group call are roaming together (for example, in the case of a group of policemen moving in the same convoy of vehicles), the MTs forming the group decide together to initiate a call re-establishment, so provoking a burst of random access requests prejudicial to the stability of the slotted ALOHA access channel. On the other hand, the response times by the network can be considerable, above all in the case described above, and create a situation that is uncomfortable, if not dangerous, for the users who remain unreachable for a few instants.
Document U.S. Pat. No. 6,292,670 discloses a method for maintaining a group call whereby, in response to the allocation of a traffic channel for the group call in a determined cell, a traffic channel is allocated for the group call in each cell adjacent to said determined cell. When a mobile terminal involved in the group call changes cell from said determined cell to a given adjacent cell, it switches to the traffic channel allocated to the group call in said adjacent cell. This technique does, however, involve a somewhat inefficient use of the radio resources of the system, given that a lot of traffic channels need to be reserved with a very low probability of being used. This waste of radio resources is that much greater when the MTs involved in the group call are not necessarily in the same cell, or in cells adjacent to each other.